The Building Regulations recognize that sometimes it is simply too expensive to refurbish an old building to modern energy standards. In these cases, the building should be upgraded to the best standard possible, such that the energy savings repay the cost of refurbishments within 15 years.
The TLX Gold standard solutions contain a range of roof structures incorporating TLX Gold, and provide an estimate of the payback period. These estimates use the following method:
The roof is being re-roofed from the outside, for structural reasons or to replace broken slates or tiles.
The simplest re-roofing option is to fit a breathable roof tile underlay to the roof. A better option is to install TLX Gold as a combined multi-foil insulation and breathable underlay.
The cost of the refurbishment is taken as the difference in cost between fitting the standard underlay and that of fitting TLX Gold and any additional insulation. Any costs that are common to both methods, such as scaffolding and re-tiling are not included.
The energy consumption of a typical semi-detached house is calculated using SAP, based on the U value of the roof when the simple breathable membrane refurbishment is used. The energy use is then re-calculated based on the roof U value when the TLX Gold option is installed. The difference between the two calculations gives the energy saving that results from the TLX Gold option.
The energy cost is based on central heating of the house, with a gas boiler using mains gas.
The payback period is simply the additional cost of installing the TLX Gold solution divided by the annual energy savings.
This calculation can only estimate the payback time, since the energy consumption of a house depends on the behaviour of the occupants. Nevertheless, it gives a fair assessment of the cost of installing TLX Gold, and the energy savings that might result.